Polyimides have excellent performances in heat resistance, mechanical physical properties, chemical resistance and the like so that they are widely used in an aerospace field, an electronic material field, etc. Most of them are an aromatic polyimide. Most of aromatic polyimides are insoluble in a solvent and non-thermoplastic and had a drawback in processability. Since a polyamic acid which is a precursor is soluble in an organic solvent, there is adopted a method in which a polyamic acid solution is formed in a desired shape and then imidated. But, the imidation is accompanied with desorption and evaporation of water. The temperature reaches even from 180 to 400° C. at the time of heat imidation and is far in excess of a boiling point of water. In the case of a thick film-shaped molded article, a fault regarding surface properties such as blister, etc. was easily generated. Hence, the selection of a condition at the time of molding, such as temperature setting, etc., involved a difficult aspect. Goods of a polyamic acid produced while omitting this imidation process cannot exhibit durability against heat, etc. inherent to a polyimide. Also, a polyamic acid solution is easily hydrolyzed in the presence of water, and therefore, a method for storing it included a drawback.
As one of methods for solving the foregoing problems, there is an organic solvent-soluble polyimide. As a matter of course, the organic solvent-soluble polyimide is able to be formed into a polyimide solution, and therefore, after forming into a desired shape, it can be processed only by volatilizing the organic solvent, whereby goods having good surface properties are easily obtainable. Also, it is excellent in storage stability.
As to this organic solvent-soluble polyimide, there have been made a number of studies, and they chiefly reply upon a devising of polymerization components. A method of introducing a benzophenone skeleton (see Patent Document 1) and the like are disclosed, and as one approach for this method, the introduction of an alicyclic structure is exemplified. For example, the use of isophoronediamine as a raw material is attempted (Patent Documents 2 and 3). However, in the polyimide resin obtained from isophoronediamine, goods with a high molecular weight are hardly obtainable, and in view of its skeleton, they are relatively rigid with respect to physical properties and brittle. There is a tendency that the coefficient of water absorption is high. On the other hand, there are disclosed polyimide resins containing a 1,2,4,5-cyclohexanetetracarboxylic acid skeleton (see Patent Documents 4, 5, 6 and 7). However, there are polyimide resins which have a 1,2,4,5-cyclohexanetetracarboxylic acid skeleton but which may not be said to have high solubility in an organic solvent as in those disclosed in Patent Documents 4 and 5. Patent Document 6 discloses a polyimide resin containing a 1,2,4,5-cyclohexanetetracarboxylic acid skeleton, which is ease to realize a high molecular weight and capable of easily producing a flexible film and which has sufficiently large solubility in an organic solvent. Also, Patent Document 7 discloses a method for manufacturing a solvent-soluble polyimide by polycondensation of an aliphatic tetracarboxylic dianhydride, an aliphatic tetracarboxylic acid or a derivative thereof and a diamine compound in a solvent.
However, most of the foregoing polyimide resins are those having a high coefficient of water absorption and involved problems such as the matter that they are inferior in hygroscopic dimensional stability in an application to a thin layer, etc.
In the light of the above, for the purpose of imparting solubility in an organic solvent while keeping performances which are characteristic features of a polyimide, such as heat resistance, mechanical physical properties, etc., a further devising is necessary in the kinds and proportions of a tetracarboxylic acid component and a diamine component, each of which is a polymerization component.
As an application example of an organic solvent-soluble polyimide, there is an adhesive for a metal-clad laminate. The metal-clad laminate includes one which is manufactured by bonding an insulating base material and a metal layer to each other via an adhesive or an adhesive film. For example, there is proposed a metal-clad laminate of a three-layer structure in which an insulating base material composed of an aromatic polyimide resin film and a metal layer are bonded to each other via an adhesive film (see Patent Document 8).
In a metal-clad laminate, there was encountered a problem that when the amount of a residual volatile component of an adhesive layer to be disposed between an insulating base material and a metal layer is high, blanching, blister, foaming, etc. of the adhesive layer is caused during a soldering step reaching a high temperature of 250° C. or higher, thereby noticeably hindering adhesive properties between the insulating base material and the metal layer (see Patent Document 9). Examples of this residual volatile component of the adhesive layer include water and the organic solvent which have not been removed in imidation and organic solvent removal steps during the formation of an adhesive layer or an adhesive film; water to be absorbed from the manufacturing environment; water to be absorbed at the time of dipping in an aqueous solution in an etching step; and the like. Of these, water is especially regarded as a problem. In order to solve the foregoing problem, it is desirable to decrease a coefficient of water absorption which is an index of the water content of a polyimide.    [Patent Document 1] JP-A-7-166148    [Patent Document 2] JP-A-2000-169579    [Patent Document 3] JP-A-2000-319388    [Patent Document 4] U.S. Pat. No. 3,639,343    [Patent Document 5] JP-A-2003-155342    [Patent Document 6] JP-A-2003-168800    [Patent Document 7] JP-A-2005-15629    [Patent Document 8] JP-A-55-91895    [Patent Document 9] JP-A-2001-329246